Re: [PATCH bpf-next v2 4/6] bpf, arm64: Impelment bpf_arch_text_poke() for arm64

[Xu Kuohai <xukuohai@huawei.com>]

On 4/22/2022 6:54 PM, Jakub Sitnicki wrote:
> Hi Xu,
> 
> Thanks for working on this.
> 
> We are also looking forward to using fentry hooks on arm64.
> In particular, attaching to entry/exit into/from XDP progs.
> 
> On Thu, Apr 14, 2022 at 12:22 PM -04, Xu Kuohai wrote:
>> Impelment bpf_arch_text_poke() for arm64, so bpf trampoline code can use
>> it to replace nop with jump, or replace jump with nop.
>>
>> Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
>> Acked-by: Song Liu <songliubraving@fb.com>
>> ---
>>  arch/arm64/net/bpf_jit_comp.c | 52 +++++++++++++++++++++++++++++++++++
>>  1 file changed, 52 insertions(+)
>>
>> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
>> index 8ab4035dea27..1a1c3ea75ee2 100644
>> --- a/arch/arm64/net/bpf_jit_comp.c
>> +++ b/arch/arm64/net/bpf_jit_comp.c
>> @@ -9,6 +9,7 @@
>>  
>>  #include <linux/bitfield.h>
>>  #include <linux/bpf.h>
>> +#include <linux/memory.h>
>>  #include <linux/filter.h>
>>  #include <linux/printk.h>
>>  #include <linux/slab.h>
>> @@ -18,6 +19,7 @@
>>  #include <asm/cacheflush.h>
>>  #include <asm/debug-monitors.h>
>>  #include <asm/insn.h>
>> +#include <asm/patching.h>
>>  #include <asm/set_memory.h>
>>  
>>  #include "bpf_jit.h"
>> @@ -1529,3 +1531,53 @@ void bpf_jit_free_exec(void *addr)
>>  {
>>  	return vfree(addr);
>>  }
>> +
>> +static int gen_branch_or_nop(enum aarch64_insn_branch_type type, void *ip,
>> +			     void *addr, u32 *insn)
>> +{
>> +	if (!addr)
>> +		*insn = aarch64_insn_gen_nop();
>> +	else
>> +		*insn = aarch64_insn_gen_branch_imm((unsigned long)ip,
>> +						    (unsigned long)addr,
>> +						    type);
>> +
>> +	return *insn != AARCH64_BREAK_FAULT ? 0 : -EFAULT;
>> +}
>> +
>> +int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type poke_type,
>> +		       void *old_addr, void *new_addr)
>> +{
>> +	int ret;
>> +	u32 old_insn;
>> +	u32 new_insn;
>> +	u32 replaced;
>> +	enum aarch64_insn_branch_type branch_type;
>> +
>> +	if (poke_type == BPF_MOD_CALL)
>> +		branch_type = AARCH64_INSN_BRANCH_LINK;
> 
> This path, bpf_arch_text_poke(<ip>, BPF_MOD_CALL, ...), is what we hit
> when attaching a BPF program entry. It is exercised by selftest #232
> xdp_bpf2bpf.
> 
> However, with this patchset alone it will not work because we don't
> emit, yet, the ftrace patch (MOV X9, LR; NOP) as a part of BPF prog
> prologue, like ftrace_init_nop() does. So patching attempt will fail.
> 
> I think that is what you mentioned to in your reply to Hou [1]
> 
> So my question is - is support for attaching to BPF progs in scope for
> this patchset?
> 
> If no, then perhaps it would be better for now to fail early with
> something like -EOPNOTSUPP when poke_type is BPF_MOD_CALL, rather then
> attempt to patch the code.
> 
> If you plan to enable it as a part of this patchset, then I've given it
> a quick try, and it seems that not a lot is needed get fentry to BPF
> attachment to work.
> 
> I'm including the diff for my quick and dirty attempt below. With that
> patch on top, the xdp_bpf2bpf tests pass:
> 
> #232 xdp_bpf2bpf:OK
> 
> [1] https://lore.kernel.org/bpf/d8c4f1fb-a020-9457-44e2-dc63982a9213@huawei.com/
> 


Hi Jakub,

Thanks for your testing and suggestion! I added bpf2bpf poking to this
series and rebased it to [2] a few days ago, so there are some conflicts
with the bpf-next branch. I'll rebase it to bpf-next and send v3.

[2] https://lore.kernel.org/bpf/20220416042940.656344-1-kuifeng@fb.com/

>> +	else
>> +		branch_type = AARCH64_INSN_BRANCH_NOLINK;
>> +
>> +	if (gen_branch_or_nop(branch_type, ip, old_addr, &old_insn) < 0)
>> +		return -EFAULT;
>> +
>> +	if (gen_branch_or_nop(branch_type, ip, new_addr, &new_insn) < 0)
>> +		return -EFAULT;
>> +
>> +	mutex_lock(&text_mutex);
>> +	if (aarch64_insn_read(ip, &replaced)) {
>> +		ret = -EFAULT;
>> +		goto out;
>> +	}
>> +
>> +	if (replaced != old_insn) {
>> +		ret = -EFAULT;
>> +		goto out;
>> +	}
>> +
>> +	ret =  aarch64_insn_patch_text_nosync((void *)ip, new_insn);
>> +out:
>> +	mutex_unlock(&text_mutex);
> 
> The body of this critical section is identical as ftrace_modify_code().
> Perhaps we could export it and reuse?
>

ftrace_modify_code() is defined in the arch code, and the prototypes are
not consistent across archs, so it doesn't seem appropriate to export
ftrace_modify_code() as a public interface.

>> +	return ret;
>> +}
> 



> ---
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index 5f6bd755050f..94d8251500ab 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -240,9 +240,9 @@ static bool is_lsi_offset(int offset, int scale)
>  /* Tail call offset to jump into */
>  #if IS_ENABLED(CONFIG_ARM64_BTI_KERNEL) || \
>  	IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL)
> -#define PROLOGUE_OFFSET 9
> +#define PROLOGUE_OFFSET 11
>  #else
> -#define PROLOGUE_OFFSET 8
> +#define PROLOGUE_OFFSET 10
>  #endif
>  
>  static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
> @@ -281,6 +281,10 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
>  	 *
>  	 */
>  
> +	/* Set up ftrace patch (initially in disabled state) */
> +	emit(A64_MOV(1, A64_R(9), A64_LR), ctx);
> +	emit(A64_NOP, ctx);
> >  	/* Sign lr */
>  	if (IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL))
>  		emit(A64_PACIASP, ctx);
> @@ -1888,10 +1892,16 @@ int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type poke_type,
>  	u32 replaced;
>  	enum aarch64_insn_branch_type branch_type;
>  
> -	if (poke_type == BPF_MOD_CALL)
> +	if (poke_type == BPF_MOD_CALL) {
>  		branch_type = AARCH64_INSN_BRANCH_LINK;
> -	else
> +		/*
> +		 * Adjust addr to point at the BL in the callsite.
> +		 * See ftrace_init_nop() for the callsite sequence.
> +		 */
> +		ip = (void *)((unsigned long)ip + AARCH64_INSN_SIZE);
> +	} else {
>  		branch_type = AARCH64_INSN_BRANCH_NOLINK;
> +	}
>  
>  	if (gen_branch_or_nop(branch_type, ip, old_addr, &old_insn) < 0)
>  		return -EFAULT;
> .